Right: repeatability. If I pedal 250 watts downhill, then 250 watts uphill, 250 watts in the big ring, then 250 watts in the little ring, will I really be pedaling the same power in each case?

Call me a sceptic.

Sorry, but I am not following. If the PM is calibrated, then why wouldn't 250w = 250w? That is like saying going 50mph uphill isn't the same as going 50mph on the freeway, unless I am missing something?

Another vaporware. Too bad the price point is $1500. I just wish someone would undercut the other makers and bring something that the masses would buy. Under $1000. Yes I know $1500 is cheaper than an SRM or Quarq.

Right: repeatability. If I pedal 250 watts downhill, then 250 watts uphill, 250 watts in the big ring, then 250 watts in the little ring, will I really be pedaling the same power in each case?

Call me a sceptic.

Sorry, but I am not following. If the PM is calibrated, then why wouldn't 250w = 250w? That is like saying going 50mph uphill isn't the same as going 50mph on the freeway, unless I am missing something?

Another vaporware. Too bad the price point is $1500. I just wish someone would undercut the other makers and bring something that the masses would buy. Under $1000. Yes I know $1500 is cheaper than an SRM or Quarq.

It's starting to happen. The P2M FSA Gossamer version is €840, which should --just-- get you in around $1000.

Right: repeatability. If I pedal 250 watts downhill, then 250 watts uphill, 250 watts in the big ring, then 250 watts in the little ring, will I really be pedaling the same power in each case?

Call me a sceptic.

"Sorry, but I am not following. If the PM is calibrated, then why wouldn't 250w = 250w? That is like saying going 50mph uphill isn't the same as going 50mph on the freeway, unless I am missing something?"

Different PM measure in different ways. So if by 250 watts you mean that your legs are generating that much, a Quark will give different readings than a PT and the PT readings will vary with gear choice, while the Quark readings will change if you go with Osy or Rotor rings.

Plus power isn't the be all end all. For trianing I suspect riding @ 250 wats uphill and riding at 250 watts downhill, give slightly different training effects.

Last edited by styrrell on Thu Oct 18, 2012 4:49 pm, edited 1 time in total.

With that said, my Quarq Sram S975 unit works just fine for the $1000 I paid for it slightly used. I shipped it to Quarq, for free, the calibrated it for my QRings....just cost me shipping, turnaround 3 days.

"Calibrating for Q-rings" is just an estimate. Since they measure cadence only once per pedal stroke, they need to make an assumption about the correlation of chain speed with power through that pedal stroke, which is just an assumption which might apply differently on a flat road to a climb. It's a fundamental limiting of the Quarq/SRAM hardware.

It's easy to see the average of power = tension times chain speed is not the average of tension times the average of chain speed: 1. apply tension with the chain stationary, 2. spin the chain with zero tension. In each of these phases power is zero, yet the Quarq's approach of power = average tension times average chain speed would fail.

I don't follow you. They use the weight hanging technique on the DS arm with the rings installed. How this translates to the numbers not being accurate, I'm not sure since they are likely doing it at the same point/crank arm angles as round chains. Hence, the variable is going to be within the +-2% I would imagine, the same as any other PM.

"Thus far, your options are Garmin’s Vector pedal built around the Look Keo and won’t be available until next year at the earliest (side note: this summer, they embarked on a complete redesign internally to improve durability and aren’t stating any potential dates. The next time you hear something from them, it’ll go on sale that same day) "

Power doesn't just depend on avg force and avg pedal speed during a pedal stroke: it also depends on how these vary relative to each other. Quarq makes a certain assumption about how they vary. That assumption needs to be different for round rings versus eccentric rings. But the assumptions won't apply equally well to all pedaling styles. If you're slogging through the mud or up a steep hill or into a block headwind, the variations will be different then if you're hammering a descent. In other words, if you produce exactly 250 watts, it may report more in some cases, less in others.

Powertap doesn't have this problem: it doesn't care about the details of your pedal stroke. Powertap has other issues, but ring shape shouldn't affect its accuracy at all. Neither would it affect the Garmin Vector.

I guess that I am assuming you are using the PowerMeter for training, not JRA. If that is the case I also assume that you are trying to induce a training response by loading your body repeatedly on a known course or a trainer (i.e., intervals). To me, that implies that the training interval is on a climb, with the recovery on the descent.

I agree that if you have multiple PowerMeters, the accuracy matters, which is why I recalibrate.

Finally an update from Garmin regarding the Vector. All credit to DC Rainmaker for listening to the earnings call and getting the information presented below:

Quote:

Instead the update came during the investor/media Q&A period where it was asked what the timeline status looks like on Vector.

The quoted answer from Cliff Pemble, President and Chief Operating Officer of Garmin:

"We are making progress on Vector and we've been evaluating new design changes and we do feel like things are looking promising and are sticking to our early next year first quarter (Q1CY13) or early second quarter (Q2CY13) deliveries.”

This roughly aligns with what I’ve been hearing in other circles, though this is the first time we’ve heard 2nd quarter mentioned publically. I suspect we’ll likely see more public information following Christmas going into CES2013 (first week of January) – which is a popular time for consumer electronic companies to make announcements concerning the upcoming few months.

Power doesn't just depend on avg force and avg pedal speed during a pedal stroke: it also depends on how these vary relative to each other. Quarq makes a certain assumption about how they vary. That assumption needs to be different for round rings versus eccentric rings. But the assumptions won't apply equally well to all pedaling styles. If you're slogging through the mud or up a steep hill or into a block headwind, the variations will be different then if you're hammering a descent. In other words, if you produce exactly 250 watts, it may report more in some cases, less in others.

Powertap doesn't have this problem: it doesn't care about the details of your pedal stroke. Powertap has other issues, but ring shape shouldn't affect its accuracy at all. Neither would it affect the Garmin Vector.

Well their "assumptions" are very accurate. Friend of mine has a powertap, he has used both round rings, and biopace. We have done the same race/group training rides. Then I send my Garmin file to him.

He takes them, just like I do in WKO training peaks software, overlays the files to compare. They are basically identical the entire time, the only difference is he weighs 20-25lbs heavier than me, and his power output will show about 10% higher on average due to his fat butt being hauled around. Otherwise, the curves, peaks, lows etc...all are identical. It is easy to correlate and see the route and how we are riding within 50ft of each other the entire time.

The proof is in the pudding as they say. So Quarq is doing the calculations very similar to a powertap no matter what rings on the crank once calibrated properly.

I feel like round vs oval rings will result in power differences "at the wheel". Rather than "at the crank". I don't think strain gauge deflection on the crank arm really cares what shape or size your chain rings are. Having said that, the number reported might not change, but as others have said the calibration and assumptions made by the power meter may no longer be valid. So for training purposes, switching back and forth between different chainrings may not give comparable results but at least they'll be consistent compared by itself.

But if I roll dice and say "I'll probably get doubles in the next six throws", then 12 throws later no doubles, I'd still say "I'll probably get doubles in the next six throws" and eventually I will. Unless the dice are loaded. But it takes a lot more than 12 throws to draw that conclusion.

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